Star polymers are nano-scale materials with a globular shape, multiple segmented arms and a high-density of peripheral functionality. The spherical shape and dense structure of this type of polymer are expected to provide a suite of properties and functions different from that of linear polymers. Indeed the preparation of functionalized star polymers with uniformed size and multiple arms/functionalities is presently the subject of extensive academic and industrial interest due to their unique structure and potential applications in drug delivery systems, coatings, membranes and lithography.
Synthesis of star polymers is most often accomplished by “living” polymerization techniques via one of three strategies:                1) “core-first” which is accomplished by growing arms from a multifunctional initiator;        2) the “arm-first” method which involves cross-linking preformed linear arm precursors using a divinyl compound, or        3) “coupling-onto” involving attaching preformed arms onto a multifunctional core.        
The “core-first” method is exemplified by the use of a multifunctional initiator in a living polymerization process, [U.S. Pat. Nos. 5,763,548 and 6,627,314], most often employing living ionic polymerization systems. This approach is also called the “grafting from” approach where the arms of the star are grown from a preformed functionalized core molecule or particle. As noted above the “arm-first” strategy can be sub-categorized according to the procedure employed for star formation. One method is chain extension of a linear arm precursor with a multivinyl cross-linking agent, and the other is coupling linear polymer chains to a multifunctional linking agent, or “grafting-onto” a multifunctional core with complementary functionality.
The development of living/controlled radical polymerization has revitalized the field of star polymer synthesis, especially for functional star polymers. Various star polymers with many arms have been synthesized, mostly using these two “arm-first” methods. [U.S. Pat. Nos. 6,512,060 and 6,627,314] However while the reaction of linking living linear polymers, or macroinitiators (MIs), by extending the linear polymer chain through incorporation of divinyl monomer units is a ready and effective means of preparing star polymers with many arms, the number of arms in the star macromolecule prepared by macroinitiator arm-first approaches have a statistical distribution of arms in each star, and additionally undergo star-star coupling reactions throughout the reaction affording star polymers with a broad molecular weight distribution (MWD). The incomplete consumption of linear polymers and the less-controlled star structure forces one to conduct an expensive fractionation step in order to obtain a star polymer with a narrow MWD.
Indeed the “arm-first” approach to star synthesis has been the subject of all living polymerizations systems. Anionic polymerization was described by Rempp [Zilliox, J. G., P. Rempp, et al. (1968). “Preparation of star-shaped macromolecules by anionic copolymerization.” Journal of Polymer Science, Polymer Symposia No. 22 (Pt. 1): 145-56], and cationic has been summarized by Kanaoka. [Kanaoka, S., N. Hayase, et al. (2000). “Synthesis of star-shaped poly(vinyl ether)s by living cationic polymerization: pathway for formation of star-shaped polymers via polymer linking reactions.” Polymer Bulletin (Berlin) 44(5-6): 485-492; Shibata, T., S. Kanaoka, et al. (2006). “Quantitative Synthesis of Star-Shaped Poly(vinyl ether)s with a Narrow Molecular Weight Distribution by Living Cationic Polymerization.” Journal of the American Chemical Society 128(23): 7497-7504]
Recently controlled/living radical polymerization processes (CRP) have been developed. Atom Transfer Radical Polymerization (ATRP), nitroxide mediated polymerization (NMP), reversible addition fragmentation chain transfer (RAFT) and catalytic chain transfer (CCT) are examples of CRP processes that provide a relatively new and versatile methods for the synthesis of polymers with controlled molecular weight, low polydispersity and site specific functionality. Indeed, since CRP processes provide compositionally homogeneous well-defined polymers (with predicted molecular weight, narrow molecular weight distribution, and high degree of α- and ω-end-functionalization) they have been the subject of much study as reported in several review articles. [Matyjaszewski, K., Ed. Controlled Radical Polymerization; ACS: Washington, D.C., 1998; ACS Symposium Series 685. Matyjaszewski, K., Ed.; Controlled/Living Radical Polymerization. Progress in ATRP, NMP, and RAFT; ACS: Washington, D.C., 2000; ACS Symposium Series 768; Matyjaszewski, K., Davis, T. P., Eds. Handbook of Radical Polymerization; Wiley: Hoboken, 2002; Qiu, J.; Charleux, B.; Matyjaszewski, K. Prog. Polym. Sci. 2001, 26, 2083; Davis, K. A.; Matyjaszewski, K. Adv. Polym. Sci. 2002, 159, 1.] Therefore controlled radical polymerization procedures have been applied to star synthesis; e.g. by Zhang. [Zhang, L. and Y. Chen (2006). “Allyl functionalized telechelic linear polymer and star polymer via RAFT polymerization.” Polymer 47(15): 5259-5266.] All of these procedures employed the reaction of “living” polymers, or macroinitiators with a difunctional monomer.
While CRP procedures have been employed to prepare many novel copolymers a need still exists for a robust, inexpensive process for the preparation of functional star macromolecules with control over all aspects of the synthesis of multi-arm star macromolecules.
Recently, a new strategy for the high-yield synthesis of low-polydispersity star polymers was reported. This was accomplished via copolymerization of a linear macromonomer (MM) and divinyl cross-linker using a low molar mass ATRP initiator in a homogeneous solution. [Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614; Matyjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921; Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689] The procedure was called the MM method. [Gao, H.; Ohno, S.; Matyjaszewski, K. J. Am. Chem. Soc. 2006, 128, 15111; Gao, H.; Matyjaszewski, K. Macromolecules 2007, 40, 399] In this method, the incorporation of arms into a star depends on the copolymerization of a linear MM with a crosslinker, which completely bypasses the requirements associated with chain extension of linear MI's, the MI method. However this procedure requires the presence of a catalyst complex that has to be removed from the solution of the final star macromolecule prior to use in many applications.
On the other hand, conventional free radical polymerization (FRP) possesses a number of advantages compared to any other chain-growth polymerization techniques, including easily attainable experimental conditions that are applicable to a broad range of monomer species. The products of a FRP do not possess any undesired functional groups such as halides present in many ATRP materials or sulfur containing degenerative transfer agents present in RAFT systems. However, FRP has intrinsic limitations, such as, slow initiation due to slow degradation of an added free radical initiator resulting in continuous generation of new chains and inevitable termination of all radicals during the polymerization, which has excluded the possibility of using the multifunctional initiator method and/or cross-linking of preformed macroinitiators (MIs) method for synthesis of well defined star polymers by FRP.
Furukawa and Ishizu reported the first approach to overcome these limitations associated with synthesis of star-like polymers by FRP of a polyisoprene MM and divinylbenzene (DVB). [Furukawa, T. and Ishizu, K (2002). Journal of Colloid and Interface Science, 253(2), 465-469] The solvent for the copolymerization was selected to induce micellization of the linear MM's, and solubilize the thermal initiator and DVB in the core of the micelle surrounded by the polyisoprene arms. Subsequent in-situ FRP occurred within each isolated core thereby stabilizing the micelle structure and producing core cross-linked star polymers. Therefore the process required pre-formation of micelles that were the precursor of the stars and there was no control over the polymerization process that occurred within the micelles and functional star core macromolecules were not prepared. Indeed due to the low efficiency of micellization, the star yield was low (<40%) and most of the linear MMs were not incorporated into star copolymers after polymerization. As described herein, the process of the present disclosure does not require micellization and allows incorporation of a much higher fraction of the added linear macromonomers, greater than 50%, preferable greater than 75% and optimally greater than 85%.
Another approach was disclosed by Antonelli, et al. in U.S. Pat. No. 5,362,813 when they conducted a free radical copolymerization of macromonomers prepared by cobalt mediated catalytic chain transfer polymerization with a diacrylate. This can be considered a crosslinking reaction involving a polyacrylate macromonomer possessing an α,α-disubstituted olefin, wherein one of the substituents is a carboxylic ester, and while star yield was not reported the polydispersity of the formed stars varied between 2.63 and 8.0, showing poor control over the star formation reaction.
We disclose a process that overcomes these limitations. The invention is not limited to the specific compositions, components or process steps disclosed herein to exemplify the concept as such may vary.
The term “multi-arm star” indicates that a star shaped macromolecule with three or more arms linked at the core of the star is formed.
The term “mikto-arm star” indicates that one or more of the arms in the multi-arm star macromolecule has a different composition than other arms attached to the same star core.
It is also to be understood that the terminology used herein is only for the purpose of describing the particular embodiments and is not intended to be limiting.
The procedure for preparation of star copolymers is exemplified by (co)polymerization of linear macromolecules, including macromonomers (MMs) with a divinyl cross-linker.
In one embodiment of the invention the MM used in this reaction can further comprise a reactive or a responsive functionality.
In another embodiment the added free radical initiator comprise a second functionality that is thereby incorporated into the core of the star macromolecule.